The present invention relates generally to photodetectors and more specifically to techniques for providing a linear detector output for use in a Fourier transform spectrometer.
A Fourier transform spectrometer typically includes a Michelson interferometer into which an infrared beam to be analyzed and a monochromatic reference beam (typically in the visible range) beam are directed. The interferometer has a fixed mirror and a movable mirror which is driven at a nominally constant velocity over a portion of its travel. Each of the input beams is split at a beam splitter with one portion traveling a path that causes it to reflect from the fixed mirror and another portion traveling a path that causes it to reflect from the movable mirror. The portions of each beam recombine at the beam splitter, and the recombined infrared and monochromatic beams are directed to appropriate photodetectors (detectors).
Due to optical interference between the two portions of each beam, the intensity of the monochromatic beam is modulated at a frequency proportional to its optical frequency and the mirror velocity while each frequency component of the infrared beam is modulated at a frequency proportional to that component's optical frequency and the mirror velocity.
Each detector has associated circuitry to generate a voltage representative of (preferably proportional to) the light intensity falling on the detector. The infrared detector output signal therefore represents the superposition of the modulated frequency components and provides an interferogram whose Fourier transform yields the desired spectrum. The monochromatic detector provides a nominally sinusoidal reference signal whose zero crossings occur each time the moving mirror travels an additional one quarter of the reference wavelength. The data acquisition electronics are triggered on these zero crossings to provide regularly sampled values for the interferogram. With the appropriate choice of mirror velocity, the output signal can be made to fall within a convenient range of modulation frequencies, as for example in the audio range.
Certain types of detectors, such as photodiodes and photomultiplier tubes are typically used with DC amplifiers, while others, such as photoconductors are typically used with AC amplifiers. In either case, the average value of the interferogram provides no useful spectral information, and is typically subtracted out before performing the Fourier transform.
The interferogram is characterized by a centerburst region of very large intensity fluctuations, corresponding to the portion of the mirror travel where the two optical path lengths in the interferometer are equal. The centerburst need not be at the center of the interferogram since the mirror need not be scanned equal distances on either side. A technique called gain ranging is sometimes used to lower the amplifier gain in the centerburst region.
One type of infrared detector comprises a photoconductive material such as mercury cadmium tellurium (MCT). A photoconductive material has a conductivity that varies with the luminous flux falling on the material. The ideal photoconductor would have a linear characteristic, so that when it is biased with a constant voltage, the current through it would vary linearly with the intensity. It is well known to use negative feedback in the preamplifier to provide an output voltage that is a direct measure of the detector current.
The real-world photoconductor is not linear, however, but rather exhibits non-linear behavior that to some extent approximates that of an ideal photoconductor in series with a fixed resistance. In the context of a Fourier transform spectrometer, the non-linearity in the detector output signal manifests itself by distorting the resultant spectrum in the wavelength regions where the detector is sensitive, and producing artifacts indicating the presence of energy in wavelength regions where the detector is actually insensitive. A typical approach is to accept the non-linearity as inevitable, and operate in a range of low infrared source intensity where the detector characteristic is approximately linear. This is sometimes undesirable, however, since a greater source intensity would improve the signal-to-noise ratio of the spectral measurement.
It is also known to correct the non-linearity by providing positive feedback in the preamplifier circuit so as to balance out the non-linear effect of the internal series resistance. While the positive feedback approach is presumably effective in correcting the detector non-linearity, the use of positive feedback will increase the effective noise of the amplifier. This is not a problem for relatively noisy detectors where the detector noise (as opposed to the amplifier noise) is the limiting factor in the overall signal-to-noise ratio. However, current state of the art detectors are characterized by low noise; therefore, amplifier noise becomes a performance-limiting factor. For example, if the detector noise is only on the order of twice the preamplifier noise, the effect of positive feedback in the preamplifier will be to raise the preamplifier noise to the same level as the detector noise.